Technical Abstract:
There is an increased interest in the United States in grafting watermelon on cucurbit rootstocks to control soilborne diseases. Several cucurbit species including Lagenaria siceraria, Cucurbita spp. and Benincasa hispida (wax gourds) have been used in Asia as rootstocks for watermelon. In our preliminary field experiments in Charleston, SC, the cucurbit species Praecitrullus fistulosus was also found to be compatible for grafting watermelon. The P. fistulosus has been considered a close relative of watermelon (Citrullus lanatus var. lanatus), and previously was classified as Citrullus lanatus var. fistulosus (Stocks) Duthie & J.B. Fuller. Still, different studies indicated that it’s taxonomic classification in relation to watermelon is incomplete. In this study, we used EST-PCR, EST-SSR and SRAP markers to assess phylogenetic relationships of P. fistulosus with Citrullus spp. (C. lanatus var. lanatus, C. lanatus var. citroides, C. colocynthis), Cucumis spp. (including C. melo, C. sativus, C. anguria, C. meeusei, C. zeyheri), B. hispida, L. siceraria and Cucurbita spp. (including C. pepo and C. maxima). The data indicate that P. fistulosus is phylogenetically closer to B. hispida and L. siceraria than to Citrullus, Cucumis and Cucurbita species. In an additional study, phylogenetic relationships were determined among United States Plant Introduction accessions (PIs) of L. siceraria collected throughout the world. Two major groups of L. siceraria PIs were identified. The first group included L. siceraria PIs collected in Asia, while the second group included PIs collected in Central and South America. Several of the L. siceraria PIs collected in Asia are resistant to zucchini yellow mosaic virus (ZYMV), while several of the PIs collected in Central and in South America showed lower galling in response to root-knot nematodes (RKN). In field experiments in Charleston, SC, several of the L. siceraria PIs that are resistant to ZYMV or moderately tolerant to RKN were highly compatible for grafting watermelon, and promoted watermelon fruit quality.